I hear that deleting user accounts, posts and threads can seriously muck up the database if not done properly (not sure about this, but it's what I've been told).
But you're right, instead of locking old threads as they're resurrected, for purely no reason, why David or a mod doesn't just use the Mod CP to do a mass lock job on posts over a certain age. It could be done once a month or something like that.
Don't know if vBulletin supports mass locking, but it would be an ideal solution.